1. Technical Field
The invention is related to plasma reactors for processing semiconductor integrated circuit wafers and specifically to improvements in the gas injection system employed in such reactors.
2. Background Art
A plasma reactor for processing semiconductor integrated circuit wafers typically includes a vacuum chamber, a pedestal for supporting the wafer in the chamber, a plasma RF power source and a gas injection system for supplying gases to the chamber. If the reactor is an inductively coupled reactor, then it can include a coil antenna around the chamber connected to the plasma RF power source. The wafer pedestal can also be connected to the same or another RF power source. In other types of plasma reactors (such as, for example, a reactive ion etch reactor), there is no coil antenna and the plasma RF power source is connected solely to the wafer pedestal. The gas injection system of the reactor has one or more gas distribution apparatuses. If multiple gas distribution apparatuses are employed, each is typically disposed in a separate part of the reactor so as to provide gas to a different area within the chamber. The gas distribution apparatus (or apparatuses) utilized depends on the particular requirements of the process being performed. For example, one type of gas distribution apparatus is used to inject gas radially into the chamber from the reactor's sidewall, typically near the level of the wafer, during various processing operations (e.g. plasma enhanced chemical vapor deposition). This radial gas distribution apparatus may be used alone, or in combination with other gas distribution apparatuses, such as the aforementioned overhead type.
One typical radial gas distribution apparatus 10 is shown in FIG. 1. This apparatus 10 is located around the periphery of the base or bottom of a source region 12 of the chamber. The apparatus 10 includes a gas distribution ring 14 having gas injection holes 16 spaced periodically around the inward facing surface of the ring 14. Processing gas, or the like, is fed into the ring from a single inlet 18. Although sufficient for its designed task, the prior art radial gas distribution apparatuses 10 do have the disadvantage of requiring a considerable portion of the chamber to be physically dedicated to the gas distribution ring structure 14. In current plasma reactor designs there is a desire to package the reactor's numerous systems in as compact a unit as possible. Thus, giving up the space necessary to incorporate a large, intrusive gas distribution ring 14 presents a problem. In addition, the interface 20 between the reactor sidewall 22 and the gas distribution ring structure 14 is typically sealed to prevent gases from leaking through the interface. To accomplished this task, a relatively large sealing element (not shown) must be employed due to the extensive sealing surface area associated with the interface 20. As is well known, sealing large surfaces presents some difficulty, and leakage is a concern. For example, the dimensional tolerances associated with the surface against which a sealing element, such as a sealing O-ring, is seated must be relatively precise to prevent leak paths from forming. The larger the sealing surface, the more difficult it is to produce the required tolerances over its entire area.
Another concern associated with the interface between the reactor chamber sidewalls 22 and the relatively large prior art gas distribution rings 14 is the discontinuity the interface 20 creates in the wall surface. These discontinuities create stress points and fractures in the layer of deposition residue which inevitably forms on the chamber sidewalls 22. In addition, temperature gradients tend to form between the sidewalls 22 and the ring 14, further increasing the occurrence of stress fractures in the residue layer. The fractured residue material tends to flake off the ring 14 and sidewalls 22, thereby creating a risk of contaminating a wafer undergoing processing. This flaking phenomenon is particularly prevalent near the gas injection holes 16 where the turbulence caused by the incoming processing gas tends to disperse the flaked residue material into the chamber.
Asymmetric gas flow from the distribution ring 14 is also a problem encountered with existing radial gas distribution apparatuses. Since the gas distribution ring 14 is typically fed from one location, the gas flow from the gas injection holes 16 near that input can be much larger than at the opposite side of the ring. Such an asymmetric flow pattern can cause nonuniform etching or deposition on the semiconductor wafer. Also, with only one feed, if the gas is a composite of different gases, it must be premixed before it reaches the ring 14. There is no possibility of providing different gases from the various holes 16 in the ring 14 so as to allow mixing within the chamber or varying the composition of the gas in different parts of the chamber.
Thus, there is a need for a plasma reactor gas injection system which requires less space and has a smaller sealing surface area than currently available units. In addition, there is a need for a gas injection system which has the ability to provide a symmetrical gas flow to the chamber, and which can simultaneously deliver different gases to the chamber for mixing therein or varying the composition of the gas in different parts of the chamber.